Create per-device sysctl nodes that can be used to query the current state of the UART. This could include the line settings (e.g. baudrate, databits, stopbits and parity) as well as the current modem status (e.g. DTR, DSR, RTS, CTS and DCD signals).
pc98 support is still not going anywhere.
It would be nice if uart(4) could support USB based UARTs.
References
http://home.jp.freebsd.org/~nyan/serial/i8251_pc98.txt (This link is broken)
http://home.jp.freebsd.org/~nyan/serial/ns8250_pc98.txt (This link is broken)